Headspace
In firearms terms, the headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt. Used as a verb, headspace refers to the mechanism by which the correct positioning is achieved. Different cartridges have their datum lines in different positions in relation to the cartridge. For example, 5.56 NATO ammunition headspaces off the shoulder of the cartridge, whereas .303 British headspaces off the rim of the cartridge.
If the headspace is too short, ammunition that is in specification may not chamber correctly. If headspace is too large, the cartridge case may rupture, possibly damaging the firearm and or possibly injuring the shooter.
[edit] Headspace gauges
Headspace is measured with a set of two headspace gauges; a "go" gauge, and a "no-go" gauge. Modern headspace gauges resemble the cartridges for the chambers they are designed to headspace, and are typically made of heat-treated tool steel. Both a "go" and a "no-go" gauge are required to headspace a firearm properly.
Headspace gauges are typically used by inserting the gauge into the firearm chamber. The bolt should close and lock on a "go" gauge, and not close on a "no-go" gauge, indicating that the chamber headspace of a firearm is within safe minimum and maximum dimensions, respectively. The force that is applied to the bolt on a bolt-action firearm when making these assessments should only be at normal levels of force; otherwise, an incorrect assessment of headspace may result if the bolt is forced into a position with excessive pressure.
For current or former military calibers, a "field" gauge can also be used. The "field" gauge is designed to take the place of the "no-go" gauge in military firearms, and functions in the same way. Military firearms are designed to withstand higher pressures. As such, a greater tolerance in the firearm's headspace is acceptable, and the "field" gauge takes into account this greater tolerance. "Field" gauges should be used only on military firearms, and not on commercial firearms. Headspacing a commercial firearm with a "field" gauge can create an unsafe condition.
As the "field" gauge takes the place of a "no-go" gauge, any military-surplus gun that locks on a "field" gauge is unsafe to fire, and should be checked by a trained gunsmith.
Guns that fail to lock on the "go" gauge may simply need cleaning, especially at the bolt face, as build-up may occur on this surface and this buildup can cause problems in chambering a round without stressing the brass.
Headspace gauges are designed to indicate simply whether a firearm's chamber is in tolerance. There can be other types of gauges that measure exactly how over or undersize a chamber might be.
